1. Field of the Invention
The present invention relates to a radio communication system for carrying out communications of audio signals using radio signals.
2. Description of the Related Art
Recently, in radio telephone apparatuses for communicating audio signals by radio, digital communication using a TDMA-TDD system such as a DECT system or PHS system has been made main stream. And, in the IMS band, TDMA-TDD system communication apparatuses have been developed, which are composed of a frequency hopping system for carrying out communications while gradually varying the use frequencies.
It becomes an important subject to prevent communication quality from deteriorating due to radio interference when implementing radio communications. In communication apparatuses of a TDMA system using a frequency hopping system, a method for preventing errors from occurring due to radio interference, in which a so-called channel-swap method is employed, has been generally utilized. In the channel-swap system, a part of the channels that can be utilized for communications are made into reserve channels for substitution, and where a radio status is favorable, channels other than the reserve channels are used for communications while swapping the channels (frequencies) in a predetermined order. As the radio quality of a channel in use is worsened, a notice is issued to a communication partner, which instructs substitution of the worsened channel for a reserve channel, and the communication is continued.
However, in the channel swap, since channels are substituted after radio deterioration occurs, it was difficult to prevent an initial receiving error. Also, where only a few reserve channels are available, there is another problem in that all the channels whose radio status is not satisfactory cannot be substituted for reserve channels. On the other hand, where the number of reserve channels are increased, the number of channels (frequencies) that are used for communications is decreased, and the band of frequency diffusion by frequency hopping is narrowed. This brings about a result which runs counter to the inherent purpose of widening the frequency diffusion by frequency hopping. Therefore, it becomes difficult to sufficiently prevent interference.
Further, as a communication system which is further less influenced by radio interference, such a system has been proposed, in which a radio link having a favorable communication status is selected among a plurality of radio links established, and attempts to improve the communication quality. In a radio communication apparatus of a TDMA system using the present method, signals that are obtained by digitally converting audio signals to be transmitted are transmitted by a plurality of slots in one frame, and data whose receiving condition is favorable are selected at the receiving side from the receiving data in a plurality of receiving slots in one frame, and the audio signals are re-constructed therefrom. With respect to the reference of selection, a method for selecting data in slots having no error detected using CRC 124 data for judging errors, which are transmitted along with the audio data, that is, having no CRC 124 error detected is easily achievable.
In Japanese Published Examined Patent Application No. Hei-7-28248 that is a Japanese Patent Publication Gazette, frequency diversity is introduced, in which all the signals in a frame are made identical to each other and the same signals are sent with the transfer frequencies varied in terms of time in a system (frequency hopping) for carrying out communications while varying frequencies for respective slots (one of divided periods of time in a frame is made into a slot) in one frame using a plurality of frequencies in a time division radio communication system. In addition, this is called a slot diversity technique in that not only are the frequencies varied, but also the same signals are transmitted by a plurality of slots in one frame.
However, in a method by which the same data are merely transmitted a plurality of times by a plurality of slots (or all slots) in one frame and signals in favorable slots are selected, there is a problem in that a number of radio resources are consumed in comparison with a case of carrying out communications by only one radio link. In particular, in the TDMA communication using the frequency hopping system, consumption of radio resources becomes a great problem in a communication apparatus for preventing interference by channel swap, because, in the TDMA communication using the frequency hopping system, since it is difficult to substitute channels in response to the receiving conditions of all receiving partners with respect to broadcast signals used for control in a communication apparatus for preventing interference by channel swap, it was impossible to carry out channel swap with respect to the broadcast signals used for control. Therefore, since it is difficult to prevent the radio quality from deteriorating even if it occurs where audio signals for service are overlapped on communication slots of broadcast signals used for control, the communication slots for control cannot be commonly used for communication service. Accordingly, in the TDMA communication apparatus for preventing interference by channel swap using the frequency hopping system in which one of the usable slots is occupied for control, an increase in the number of slots used for a single communication service simultaneously results in a lowering in the number of communication-possible slots, where it remarkably adversely influences communications.
Also, since a plurality of radio links are established at all times when carrying out communications in a conventional method for establishing a plurality of radio links and for selecting signals of a radio link having a favorable state, radio resources which can be used, that is, only a half or less the number of slots can be simultaneously used in the TDMA system.
In addition, in order to minimize a lowering in the radio resources, such a method has been proposed, by which, where deterioration of radio signals is detected, a new radio link is established, and signals of a radio link whose receiving state is favorable are selected and are re-constructed, and when deterioration of the radio signals is improved, one radio link is left over and the other radio links are opened. However, the receiving state is not favorable until a plurality of radio links are established after a radio-deteriorated state is recognized. Also, in an environment where the radio quality always deteriorates, it is impossible to avoid deterioration of the communication quality in commencement of communications.
Further, even if, in conventional slot diversity means, it is possible to avoid a communication failure as a system when a sequence collision occurs, communication is impossible in one communication slot in which the sequence collision has occurred, wherein a diversity effect is lost.